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Southern Cross - May 1998

• Constellation of the Month - Antlia
• A Small Group of Galaxies in Antlia
• Beta Centauri as a Double Star

Constellation of the Month - Antlia

Albert Brakel

Antlia the air pump straddles the boundary of the Milky Way between Vela and Hydra. It was invented by Lacaille in 1752 as Machine Pneumatique, latinized to Machina Pneumatica, then Antlia Pneumatica, and finally just Antlia in 1930. Despite its position on the outskirts of the Milky Way, it contains no nebulae, but features several galaxies, most of them quite faint. None of its stars have common names, its Alpha is only mag. 4.3, and its Beta was lost long ago to Hydra.

Variable star

Of special interest to observers of variables is S Antliae (RA 9h 32.3m, Dec -28 38'), an eclipsing variable that once had the shortest known period of 7h 46m 48s, until U Pegasi was discovered to have a period of only 5.5h. It ranges from mag. 6.4 to 6.9.

Double stars

Two of the binaries in Antlia are suitable for small telescopes. Zeta-1 Ant (09h 30.8m, -31 53') is a white pair of mags. 6.2 and 7.0, and a wide 8.0" apart. Delta Ant (10h 29.6m, -30 36') also appear whitish (mags. 5.6 and 9.6), although the spectral type of the faint companion (F8) suggests it would appear yellowish if it were brighter (a large aperture will help here). Such a markedly unequal pair is usually difficult to resolve, but this one fortunately has a very wide separation of 11".

Galaxies

NGC 2997 (09h 45.6m, -31 11') in western Antlia has been made famous by David Malin's popular photograph of it, and it became the first, and so far only, galaxy to appear on an Australian postage stamp, in the International Space Year issue of 1992. Although of mag. 9.4, its light is spread over about 5', making it faint in modest instruments. In my observation with a C8 from suburban Downer, it appeared as a barely perceptible dim glow, and using higher powers or a broad-band nebular filter to increase contrast did not help much. In larger apertures from a dark site a small bright nucleus will be visible, and you may just be able to recognize it as a face-on spiral.

The remaining galaxies in Antlia , and there are heaps, are even fainter. With the almost endless overcast skies we have been having lately, my first and only chance to try to find some of them for this article was around Full Moon. Not surprisingly, this was unsuccessful. However, large 'scope owners wanting to track some down could have a go at NGC 3087, 3223, 3224, 3250, 3271 and 3347, and you can find descriptions of a group of galaxies around NGC 3250 in the following article by Jenni Kay.

A Small Group of Galaxies in Antlia

Jenni Kay, FRAS

This small group of galaxies centred on NGC 3250 is easily located, being a short three degrees star hop from the very bright naked eye star q Velorum. While searching and studying each galaxy in this group, I found that low magnification did not show these galaxies at all, in fact, while moderate power could pick most of the galaxies out in the field, it was high power which offered the best view.

For each galaxy I made my own measurements estimating their sizes. When these sizes are compared to that given in the modern catalogues there is an obvious discrepancy. My measurements are based on what I can see through a 12.5-inch telescope under the relatively dark skies of my home site. The modern catalogue sizes were derived from measuring galaxy images on plates taken with the UK Schmidt Telescope, and in most cases will include galactic halos which extend far beyond what can be seen in an amateur telescope. The co-ordinates and data given below with each galaxy has been taken from a selection of various modern catalogues, and represents the best details know to me at this time.

The following observations were made from my semi-rural home site at Lobethal, South Australia. The typical skies here offer a limiting naked eye magnitude of 6.1, ANT II-III. The instrument used was a 12.5" f/5 Newtonian. Moderate power used here was with a 15mm Televue Wide Field eyepiece at 105X, with a 37' field of view. High power was attained with a 10.5mm Plossl at 151X, with a 20' field of view.

ESO 317-27 : 10 27 00.4 -40 01 31
13.6v; 1.5' x 0.4'; SB 12.9v; PA 151d.
105X: the galaxy appears as a faint, fat, fuzzy star. 151X: the galaxy is more pronounced, seen as a faint, slightly elongated, bit of a hazy smudge. The galaxy is 0.4' x 0.2' in size, with the major axis aligned NW-SE.

VC 92 : 10 26 10.5 -39 56 35.6
105X: About 4' W of NGC 3250 the eye is drawn to an object appearing as a faint, fat, fuzzy star. 151X: the galaxy is very small, only ~ 10" in size, round, and comparable in brightness to a 14 mag field star.

ESO 317-33 : 10 28 58.6 -40 22 10
mag 16.1v; 0.6' x 0.3'; SBmag 14.1v; PA 11d.
105/151X: Too faint, nothing seen.

ESO 317-39 : 10 31 00.2 -40 10 44
16.4v; 0.9' x 0.2'; PA 168d.
105/151X: Too faint, nothing seen.

ESO 317-42 : 10 31 32.9 -39 33 33
13.0v; 1.1' x 0.7'; SB 12.6v; PA 61d.
105X: quite stands out, appearing as a relatively bright, very small, almost stellar, round hazy spot. 151X: using averted vision, a much fainter halo can be seen wrapped around a bright stellar like core. Overall, the galaxy is 0.4' in size, with the very small core being as bright as a 14 mag field star.

ESO 317-45 : 10 31 47.7 -39 45 39
14.1v; 1.5' x 0.4'; SB 13.4v; PA 138d.
105X: too faint. 151X: the galaxy can be picked out, its appearance borders on being a very faint hazy smudge to a round, fat, fuzzy star. In being stellar-like it equals the brightness of a 14 mag field star. The galaxy is a little elongated, NW-SE, and is 0.3' long. This very faint galaxy is easy to catch in brief glimpses several times for a confident sighting.

MCG-7-22-19 (ESO 317-36) : 10 29 32.2 -39 50 29
12.5v; 1.1' x 0.7'; SB 12.1v; PA 70d.
105X: the galaxy catches the eye appearing as a faint, small, hazy spot. 151X: the galaxy is more pronounced. It has an overall oval shape with a stellar nucleus, and a size ~ 0.4' x 0.3'.

NGC 3244 : 10 25.5 -39 50
12.4v; 2.2' x 1.7'; SB 13.7v; PA 165d.
In this field lie two obvious hazy glows, the first in the west being the galaxy NGC 3244, the second is NGC 3250. 105X: NGC 3244 appears as a relatively bright, oval haze,even surface brightness and nearly 0.5' long. The whole view is most attractive with the two hazy glows set in a very starry field.

NGC 3250 : 10 26.5 -39 57
12.4v; 2.2' x 1.7'; SB 13.7v; PA 165d.
105X: About 14' southeast of NGC 3244 lies this bright, round glow, which has a little brighter centre.151X: the galaxy shows a bright core with a size of 0.3', with a little softer halo around it. Overall, the galaxy is 0.5'.

NGC 3250A : 10 27.9 -40 05
14.8v; 1.2' x 0.3'; SB 13.5v; PA 87d.
A real challenge! 151X: little more than a ghostly smudge or a faint, fat, fuzzy star. Being almost stellar-like, it can be likened to a 15 mag field star for brightness. The galaxy is about 0.3' in size. The faint apparition comes and goes, but can be seen several times for a confident sighting.

NGC 3250B : 10 27.7 -40 26
12.9v; 2.3' x 0.7'; SB 13.3v; PA 6d.
105X: the galaxy appears as a relatively faint, long wisp, a little elongated, north-northeast to south-southwest, with a brighter middle. 151X: the galaxy shows to have a stellar nucleus, a very small core ~ 0.2' in size, with a fainter outer halo. Overall, the GX is 1.0' x 0.3'. In general, the galaxy stands out well in the attractive and busy star field.

NGC 3250C : 10 27.7 -40 01
13.4v; 1.8' x 0.7'; SB 13.5v; PA 56d.
105X: the galaxy catches the eye appearing as a bit of a faint stain on the background sky. 151X: It has an overall shape somewhere between oval and elongated, northwest to southeast, and a size of 0.7' x 0.3'. There is a faint, stellar point at the northern end.

NGC 3250D : 10 27.9 -39 49
13.3v; 1.9' x 0.4'; SB 12.9v; PA 27d.
105X: The eye is caught by a very faint, long wisp. 151X: this long and slender galaxy is 0.7' x 0.2' in size, with the major axis lying northeast - southwest. The galaxy shows an uneven surface brightness. The pencil thin streak is set in a very rich star field.

NGC 3250E : 10 29.0 -40 06
12.5v; 1.9' x 1.5'; SB 13.5v; PA 145d.
105X: The galaxy is obvious, appearing as a relatively faint, relatively large haze. 151X: It is a little elongated, northwest - southeast, with a size of 0.7' x 0.5'. There is a faint star at the northwestern tip, and a second faint star at the southeastern end.

NGC 3276 : 10 31.1 -39 57
13.4v; 1.0' x 0.7'; SB 12.9v; PA 68d.
105X: In the field lie two hazy glows, the first in the west is the galaxy NGC 3276, the second is NGC 3278. NGC 3276 is easy enough to spot, appearing as a relatively faint, small, little-elongated haze. The surface brightness may be uneven with a couple of stars nearly resolving. 151X: there is indeed a true field star nearby, but the galaxy is well detached from it. At the lower power the star seemed attached to the galaxy giving it the appearance of being elongated. At the higher power it can be clearly seen to be a small, round haze, about 0.3' in size.

NGC 3278 : 10 31.6 -39 57
12.3v; 1.4' x 1.0'; SB 12.5v; PA 60d.
About 5' east-southeast of NGC 3276. 105X: the galaxy is easy to see, appearing as a relatively bright, oval haze, with a brighter core. 151X: the galaxy is about 0.5' long, with the major axis lying northeast - southwest.

Beta Centauri as a Double Star

Ross Gould

Beta Centauri has been known since 1935 as a double star, discovered by Voute at the Lembang Observatory in Java. Hartung lists it, and remarks on its difficulty - "Owing to the proximity of A, good definition is essential to see it and I have found a deep neutral filter useful; 20cm will then show it clearly". At the time Hartung observed Beta in the early 1960s it was easier (!!) than in more recent years.

The Hipparcos data includes a new measure for Beta Cen - dated 1991.25, it is 0.87" at PA 234. The pair is closing, from a separation measured at 1.3" in 1935 and 1960; over the same period the PA slowly decreased from 259 degrees to 251. The rate of change in PA is faster in the post-1960 period (0.55 deg/yr compared to 0.32 deg/yr), without being rapid.

Beta Cen is not a nearby star, unlike Alpha Cen. Burnham, in his Celestial Handbook, remarks on the basis of the distance to Beta Cen being 490 ly that "the projected separation" is about 200 AU at 1.3" separation. This in itself suggests a period of many centuries, as one might notice by comparing the average separation of 23 AU for Alpha Cen (an 80 year binary. Hipparcos data gives a distance of 525 ly for Beta Cen; so this gives a little over Burnham's figure. The orbital period, based on an assumed average separation and the changes observed thus far, is likely to be over 500 years.

Despite various surveys for double stars in the south prior to Voute's discovery, Beta Cen was not found to be a pair. If we leave aside John Herschel's 1830s survey, which had a methodology that was unlikely to split Beta Cen if it was as close as at discovery in 1935, the other surveys were with smaller telescopes than the 24-inch Lembang refractor. Before the setting up of the 26.5-inch at Johannesburg in 1925, the largest southern refractors were the Sydney 11.4-inch (from 1874) and the McClean 18-inch at the Cape (from about 1897). Russell suggests the Sydney refractor was commonly used to check stars for duplicity at only 180x, despite some known pairs being examined and measured at higher power, so it would probably not have split Beta Cen if it was no wider than in 1935. The 18-inch at the Cape Observatory in South Africa, used by Robert Innes, is another story. It might be expected to have shown Beta Cen double if the separation was no less than at present. I don't know if Innes checked Beta Cen with this telescope.

For the present day observer, the new measure of Beta Cen explains why it has been so difficult to separate recently. I've tried and failed with 6 and 7-inch scopes at various times, and in recent weeks not found a steady enough night for trying with the C14 at Downer. Given the brightness of the pair, and the three magnitude difference between components (mags 0.7 and 3.7), it will be a very difficult object. If the rate of decrease in the separation is similar to the 1960-1991 period, the pair will now be roughly at 0.8" (in pa 230). This suggests telescopes of less than 30cm aperture are unlikely to show the pair even under excellent seeing conditions - and that larger scopes even on steady nights would have a better chance with a neutral density filter, as Hartung suggests.

On an internet news group, Bratislav Curcic in Victoria has commented that he was recently unable to split Beta Cen with 8- and 10-inch scopes; not surprising given the closer distance. Maybe the "C14 club", and others similarly empowered, could try it? Any successful observations are welcome. If you succeed - or don't, despite good seeing and good aperture, let me know - please indicate the seeing conditions, telescope, and magnification.